This application is based on applications Nos. 2000-227638 and 2000-258025 filed in Japan, the content of which is incorporated hereinto by reference.
1. Field of the Invention
The present invention relates to a photoelectric conversion device, in particular, to a photoelectric device using numerous crystalline semiconductor particles. The photoelectric conversion device according to this invention is utilized suitably in solar cells.
2. Description of the Related Art
Advent of a next-generation, low-cost solar cell that allows the quantity of silicon material to be small has been eagerly awaited.
Photoelectric devices using crystalline semiconductor particles that have been proposed so far are shown in FIGS. 8-13.
FIG. 8 shows a structure disclosed in U.S. Pat. No. 4,514,580. In FIG. 8, a steel substrate 41 is surrounded by an aluminum film 42 to which crushed silicon particles 43 are joined. The aluminum film 42 in the areas where the silicon particles 43 are not present has an insulator layer 44 formed thereon. The upper portions of the silicon particles 43 are formed with n type silicon portions 45 thereon. A transparent conductive layer 46 is formed over the entire surfaces.
FIG. 9 shows a structure disclosed in Japanese Patent No. 2522024. In this structure, a substrate 47 is formed with a diffusion-preventive layer 48 on which a rear face electrode 49 comprising a doping impurity for p type is formed. N type silicon particles 52 are densely deposited on the rear face electrode 49. The silicon particles 52 and the rear face electrode 49 are brought into contact and heated for alloying the silicon particles 52 and the material of the rear face electrode 49. Then, they are cooled, thereby forming p type regions 50 partially within the n type silicon particles. An insulator layer 51 is formed on the areas where the silicon particles 52 are not present, and a transparent conductive layer 53 is formed over the entire surface.
FIG. 10 shows a structure disclosed in Japanese Patent No. 2641800. In this structure, a tin layer 55 having a low melting point is formed on a stainless substrate 54. First conductive-type crystalline semiconductor particles 56 are deposited on the tin layer 55. An insulating layer 57 is formed on the areas where the crystalline semiconductor particles 56 are not present. After the upper portions of the crystalline semiconductor particles are ground, a second conductive-type amorphous semiconductor layer 58 is formed thereon. A metal electrode shaped as a strip is denoted by 59.
The structure in FIG. 11 is one disclosed in Japanese Patent Laid-Open Publication No. S61-124179. According to this structure, a first aluminum foil 62 is provided with openings into which silicon balls 63 having n type silicon surface portions 64 on the surfaces of p type silicon portions are inserted. The n type surface portions 64 in the rear faces of the silicon balls 63 are then removed. On a second aluminum foil 60 coated with an oxide layer 61, a part of the oxide layer 61 located at the rear faces of the silicon balls 63 is removed, thereby joining the silicon balls 63 to the second aluminum foil 60.
FIG. 12 discloses a method in which a high-melting point metal layer 66, a low melting-point metal layer 67, and semiconductor microcrystalline particles 68 are deposited on a substrate 65. The semiconductor microcrystalline particles 68 are fused, saturated and gradually cooled so that the semiconductor is grown by liquid phase epitaxial growth, thereby forming a liquid phase epitaxial polycrystalline thin film 71 (Refer to Japanese Patent Publication No.H8-34177).
In addition, FIG. 13. discloses a method in which a PN-junction is formed by forming a n type layer 73 that is formed by making an element, which is a n type impurity, diffused on both the front and rear surfaces of a p type substrate 72. In this method, a conductive diffusion region 74 to be connected to inside the p type layer 72 is formed by making the PN-junction penetrate the rear surface. A glass-type isolator 75 is formed around the conductive diffusion regions and baked so as to separate the PN-junction (Refer to, for example, Japanese Patent Publication No. S61-59678, and Japanese Patent Laid-Open Publication No. H10-233518).
However, in the photoelectric conversion device according to the U.S. Pat. No. 4,514,580 shown in FIG. 8, when the crushed silicon particles 43 are deposited on fused aluminum liquid heated up to its melting point 660xc2x0 C. so as to be bonded thereto, the fused liquid flows out permitting only a small part of the silicon particles 43 to be bonded, the fused liquid climbs the surfaces of the silicon particles 43 causing short circuit, or the fused liquid scatters. Such phenomena make control of the junction characteristics difficult.
In the photoelectric conversion device in FIG. 9 disclosed in Japanese Patent No. 2522024, because of the PN-junction present below the incident light, loss is large when generated carriers are collected.
According to the photoelectric conversion device shown in FIG. 10 disclosed in Japanese Patent No. 2641800, the tin layer 55 on the stainless substrate 54 is fused so as to be joined to the first conductivity-type crystalline semiconductor particles 56. However, the extremely low melting point of tin has the problem of low reliability. In addition, since the second conductivity-type amorphous conductive layer 58 is formed on the first conductivity-type crystalline semiconductor particles 56, the surfaces of the crystalline semiconductor particles 56 need to be sufficiently etched and cleaned before the formation of the amorphous conductive layer 58 in order to form a secure PN-junction. Also, the film thickness has to be thin taking the large light absorption of the amorphous conductive layer 58 into account. However, the amorphous conductive layer 58 with such a small thickness has only small tolerance to defects necessitating stricter management of the cleaning process and the production environment, which leads to high-cost production.
According to the photoelectric conversion device in FIG. 11 disclosed in Japanese Patent Laid-Open Publication No. S61-124179, when silicon balls having n type surface portions 64 covering the p type cores 63 are joined to the second aluminum foil 60 so as to form an aluminum-silicon alloy layer, aluminum diffuses into the n type surface portions 64 causing the n type surface portions 64 to be destroyed. Also, because the first aluminum foil 62xe2x80x2 has to be provided with openings into which the silicon balls are pressed so as to be joined thereto, evenness is required for the particle diameters of the silicon balls, which causes the cost to increase. In addition, because the temperature applied during the joining is below 577xc2x0 C., which is the eutectic temperature of aluminum and silicon, the junction is unstable.
The photoelectric conversion device shown in FIG. 12 has problems that the low melting-point metal 67 mixes into the first conductivity-type liquid phase epitaxial polycrystalline layer 69 deteriorating the performance, and that the absence of insulator causes leakage to occur between the layer and the lower electrode 66.
In the case of using crystalline semiconductor particles in the photoelectric conversion device shown in FIG. 13, it is difficult in terms of manufacture to form the conductive diffusion region 74 such that it is connected to the internal layer by penetrating the PN-junction. In such a case, the PN-junction itself is fused and destroyed when the substrate and the crystalline semiconductor particles are joined together.
The present invention has been devised in consideration of the problems described above, and the object thereof is to provide a low-cost photoelectric conversion device having good conversion efficiency.
A photoelectric conversion device according to the present invention comprises: a metal substrate or a substrate formed with a metal layer thereon; numerous first conductivity-type crystalline semiconductor particles deposited on the substrate; an insulator interposed among the first conductivity-type crystalline semiconductor particles; and a second conductivity-type semiconductor region formed on the upper portions of the first conductivity-type crystalline semiconductor particles, wherein a boundary part between the metal layer and the first conductivity-type crystalline semiconductor particles comprises an alloy portion comprising the metal and the semiconductor material, and a first conductive-type semiconductor region with high impurity concentration is formed in an interfacial part between the alloy portion and the first conductivity-type crystalline semiconductor particle on the side of the first conductivity-type crystalline semiconductor particle.
According to this photoelectric conversion device, owing to the formation of the alloy portion comprising the metal and the semiconductor material in a boundary part between the metal layer and the first conductivity-type crystalline semiconductor particles and the formation of the first conductivity-type region having high impurity concentration in an interfacial part between the alloy portion and the first conductivity-type crystalline semiconductor particle, the metal and the semiconductor particles can be securely joined, thereby improving the reliability of the joining condition between the substrate and the crystalline semiconductor particles. It is therefore possible to realize a photoelectric conversion device with high conversion efficiency.
A method of manufacturing a photoelectric conversion device according to this invention comprises the steps of: applying an insulator to a metal substrate or a substrate formed with a metal layer on its surface and depositing numerous first conductivity-type crystalline semiconductor particles thereon; pressing the first conductivity-type crystalline semiconductor particles from above the insulator so as to bring them into contact with the metal layer; heating the substrate and the insulator so as to form an alloy portion comprising the metal and the semiconductor material and a first conductivity-type semiconductor region with high impurity concentration in a boundary part between the substrate and the first conductivity-type crystalline semiconductor particles; and forming a second conductivity-type semiconductor region on the upper portions of the first conductivity-type crystalline semiconductor particles.
According to this method of manufacturing a photoelectric conversion device, an insulator is applied to the substrate on which numerous first conductivity-type crystalline semiconductor particles are deposited. The first conductivity-type crystalline semiconductor particles are pressed from above the insulator so as to be contacted with the metal layer, and then the substrate and the insulator are heated so that an alloy portion comprising the metal and the semiconductor material and a first conductivity-type region having high impurity concentration are formed in a boundary part between the substrate and the first conductivity-type crystalline semiconductor particles, and a second conductivity-type semiconductor region is further formed on the upper portions of the first conductivity-type crystalline semiconductor particles. Accordingly, the metal and the semiconductor material can be securely joined by this method, so that it is possible to obtain a photoelectric conversion device having high conversion efficiency with high reliability in the joining condition between the substrate and the crystalline semiconductor particles.
It is possible for the first conductivity-type and the second conductivity-type to be p type and n type, respectively. The xe2x80x9cmetalxe2x80x9d referred to above may be, for example, aluminum.
Another photoelectric conversion device according to this invention comprises: numerous crystalline semiconductor particles deposited on a substrate serving also as the electrode of one side; an insulator interposed among the crystalline semiconductor particles, and the other electrode formed above the crystalline semiconductor particles, wherein the crystalline semiconductor particle has a central portion being a first conductivity type, and a peripheral portion being a second conductivity type, the second conductivity-type peripheral portion around the junction between the crystalline semiconductor particle and the substrate having been removed by reaction between the insulator and the crystalline semiconductor particle.
This photoelectric conversion device according to the present invention is arranged such that crystalline semiconductor particles, which are previously provided with PN-junctions by having central portions being a first conductivity type and peripheral portions being a second conductivity type, are deposited on a substrate and joined to the substrate by means of alloy portions where both of them are fused, and an insulator made of a glass material is interposed among the crystalline semiconductor particles.
The structure above permits the crystalline semiconductor particles to be manufactured with low grain size accuracy, and ensures separation of the positive electrode from the negative electrode by the insulator. In this structure, it is unnecessary to form a conductive diffusion region that penetrates the PN-junction and is connected to the inner layer, and yet separation of the PN-junctions in the crystalline semiconductor particles can be carried out.
In addition, without need to bore holes in the electrodes, a stable junction can be obtained between the substrate and the crystalline semiconductor particles. The previous formation of the PN-junction allows for eased management of the cleaning process and the production environment for PN-junction formation. In addition, the light incident on the areas where the crystalline semiconductor particles are not present can be utilized.
Accordingly, this invention has enabled manufacture of a low-cost photoelectric conversion device with high conversion efficiency capable of yielding a large manufacturing margin.
Furthermore, owing to the flexibility in shape, the photoelectric conversion device of this invention has little dependence on the light incidence angle.
Structural details of the present invention are hereinafter described referring to the drawings.